Magnetic field coil and magnetic resonance imaging apparatus

ABSTRACT

Provided is an RF coil which can highly efficiently and uniformly irradiate a RF magnetic field having two or more magnetic resonance frequencies close to each other, and receive magnetic resonance signals of two or more magnetic resonance frequencies close to each other with high sensitivity and uniform sensitivity profile in an MRI apparatus. Two or more frequencies to which the coil is tuned are adjusted so as to be between resonance frequencies of series resonant circuits constituting the RF coil.

CLAIM OF PRIORITY

The present application claims priority from Japanese application JP2007-145029 filed on May 31, 2007, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic resonance imaging (MRI)apparatus, and more particularly to a RF (radio-frequency) coil fordetecting nuclear magnetic resonance images of two or more kinds ofnuclides.

MRI apparatuses are diagnostic imaging apparatuses for medical use,which induce magnetic resonance of atomic nuclei in arbitrary sectionscrossing a subject and obtain tomograms of the sections from generatedmagnetic resonance signals. As the magnetic resonance signals, those ofhydrogen nuclei (¹H) are generally used.

MRS (magnetic resonance spectroscopy) and MRSI (magnetic resonancespectroscopic imaging), which are types of magnetic resonance imagingmethods, are used as methods for measuring metabolic conditions in theliving bodies. Here, MRS is a method of measuring frequency distributionof magnetic resonance signals emitted by a substance, and MRSI is amethod of imaging on the basis of certain specific frequency componentsof magnetic resonance signals having frequency distribution. In theseimaging methods, in addition to the imaging based on magnetic resonancesignals of hydrogen nuclei (¹H), magnetic resonance signals of atomicnuclei other than those of hydrogen such as those of fluorine (¹⁹F),phosphorus (³¹P), sodium (²³Na) and carbon (¹³C) are also imaged. Inorder to simultaneously obtain images from magnetic resonance signals oftwo kinds of different atomic nuclei, the RF coil for irradiating a RFmagnetic field for exciting the atomic nuclei and detecting the magneticresonance signals is required to be tuned to the frequencies of themagnetic resonance signals of the two kinds of atomic nuclei (magneticresonance frequencies). Such a coil is called double-tuned RF coil.

As conventional double-tuned RF coils, there are those comprising a trapcircuit having a capacitor and an inductor connected in parallel andinserted into a loop of a coil, as shown in FIG. 20 (see, for example,Japanese Patent Unexamined Publication (Kokai) No. 6-242202 and M. D.Schnall et al., “A New Double-Tuned Probe for Concurrent ¹H and ³¹PNMR”, Journal of Magnetic Resonance, USA, 1985, 65, pp. 122-129). Thesedouble-tuned RF coils are intended to be tuned to two kinds offrequencies of magnetic resonance signals which are significantly differfrom each other, for example, those of ¹H and ³¹P, and they are notintended to be used for a case where frequencies to which they are tunedare close to each other. In order to realize double-tuning in thesedouble-tuned RF coils to two kinds of frequencies close to each other,the inductor and capacitor used for the trap circuit must have aninductance of 10 nH or lower and a capacitance of several hundreds pF orhigher, respectively. Since it is difficult to manufacture inductorshaving a small inductance, and such inductors also scarcely allowadjustable range, they are impractical. Furthermore, in capacitorshaving a large capacitance, it becomes impossible to ignore RF loss ofthe devices themselves at a frequency of 1 MHz or higher, and there iscaused degradation of the receiving sensitivity and transmissionefficiency of the RF coils.

As double-tuned RF coils used for a case where two kinds of magneticresonance frequencies are close to each other, there are double-tunedsaddle-type RF coils in which two saddle-type RF coils that resonate ateach frequency are perpendicularly disposed (refer to FIG. 21.) anddouble-tuned RF coils in which capacitor of a birdcage RF coil ispartially changed so that the coil should resonate at each frequency(see, for example, Peter M. Joseph et al., “A Technique for DoubleResonant Operation of Birdcage Imaging Coils”, IEEE Transactions onMedical Imaging, 1989, 8, pp. 286-294).

SUMMARY OF THE INVENTION

Since sensitivity profiles of the double-tuned saddle-type RF coils anddouble-tuned birdcage RF coils corresponding to two kinds of magneticresonance signals significantly differ from each other, the regionproviding favorable sensitivities for both kinds of the signals islimited. Moreover, the QD (quadrature detection) system which improvesthe sensitivity 1.4 times cannot be employed in these double-tuned RFcoils, and therefore sufficient sensitivity cannot be obtained.

The present invention was accomplished in view of the aforementionedsituation, and an object of the present invention is to provide atechnique for RF coils of MRI apparatuses to highly efficiently anduniformly irradiate a RF magnetic field having two or more magneticresonance frequencies close to each other and receive magnetic resonancesignals of these frequencies with high sensitivity and uniformsensitivity profile.

The RF coil for MRI apparatus of the present invention is characterizedin that the two or more kinds of frequencies to which the coil is tunedare adjusted to be between the resonance frequencies of two or moreseries resonant circuits constituting the RF coil.

Specifically, there is provided a RF coil for magnetic resonance imagingapparatus comprising a first series resonant circuit comprising a loopcoil made of a conductive material and one or more capacitors insertedinto the loop coil, a first circuit connected in parallel to the firstseries resonant circuit, and a signal processing circuit connected inparallel to the first circuit, wherein the first circuit comprises acapacitor and an inductor, and is connected in parallel with two or moreseries resonant circuits each having different resonance frequencies,the resonance frequencies of the series resonant circuits also differfrom resonance frequency of the first series resonant circuit, and theresonance frequencies of the magnetic field transmit and receive RF coilare adjusted so as to be between the resonance frequency of the firstseries resonant circuit and the resonance frequencies of the seriesresonant circuits. Here, the first circuit is a circuit connected inparallel to the first series resonant circuit and comprising seriesresonant circuits connected in parallel. Moreover, it comprises two ormore series resonant circuits of different resonance frequenciescomprising one or more capacitors and one or more inductors andconnected in parallel, and the resonance frequencies of the seriesresonant circuits also differ from the resonance frequency of the firstseries resonant circuit.

According to the present invention, in an MRI apparatus, the RF coil canhighly efficiently and uniformly irradiate a RF magnetic field havingtwo or more magnetic resonance frequencies close to each other, andreceive magnetic resonance signals of these frequencies with highsensitivity and uniform sensitivity profile.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1A and FIG. 1B are schematic views of the MRI apparatus based onthe first embodiment.

FIG. 2 is a block diagram of the MRI apparatus based on the firstembodiment.

FIG. 3 is a circuit diagram of the double-tuned loop-type coil of thefirst embodiment.

FIG. 4A, FIG. 4B and FIG. 4C are diagrams for explaining operation ofthe double-tuned loop-type coil of the first embodiment.

FIG. 5A and FIG. 5B are diagrams for explaining operation of a usualseries resonant circuit.

FIG. 6A, FIG. 6B, FIG. 6C FIG. 6D, FIG. 6E and FIG. 6F are diagrams forexplaining the method for determining the resonance frequencies f_(B)and f_(C) in the first embodiment.

FIG. 7 is a circuit diagram of the double-tuned saddle-type coil of thefirst embodiment.

FIG. 8 is a circuit diagram of the double-tuned butterfly-type coil ofthe first embodiment.

FIG. 9 is a circuit diagram of the double-tuned solenoid-type coil ofthe first embodiment.

FIG. 10 is a circuit diagram of the triple-tuned loop-type coil of thefirst embodiment.

FIG. 11A, FIG. 11B, FIG. 11C and FIG. 11D are diagrams for explainingoperation of the triple-tuned loop-type coil of the first embodiment.

FIG. 12A and FIG. 12B are circuit diagrams of the transmit and receiveRF coil of the second embodiment.

FIG. 13 is a diagram showing an example of connection of the transmitand receive RF coil of the second embodiment.

FIG. 14 is a diagram showing connection relation of the RF coil of thethird embodiment.

FIG. 15A and FIG. 15B are circuit diagrams of the double-tuned birdcageRF coil of the third embodiment.

FIG. 16A, FIG. 16B and FIG. 16C are circuit diagrams of the double-tunedloop-type coil of the third embodiment.

FIG. 17 is a circuit diagram of the double-tuned array coil of the thirdembodiment.

FIG. 18 is a diagram showing an example of connection of thedouble-tuned loop-type coil of the fourth embodiment.

FIG. 19 is a schematic view of the double-tuned loop-type coil of thefirst embodiment, which is attached with an electric wave shield.

FIG. 20 is a circuit diagram showing configuration of a conventionaldouble-tuned RF coil.

FIG. 21 is a circuit diagram showing configuration of a conventionaldouble-tuned saddle-type RF coil.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

The first embodiment of the RF coil according to the present inventionwill be explained. The whole configuration of an MRI apparatus havingthe RF coil of this embodiment will be explained first. FIG. 1A and FIG.1B are schematic views of the MRI apparatus having the RF coil of thisembodiment. In the diagrams, the direction of the z axis in thecoordinate 9 is a static magnetic field direction. The MRI apparatus 100shown in FIG. 1A is provided with a horizontal magnetic field typemagnet 101. The MRI apparatus 200 shown in FIG. 1B is provided with avertical magnetic field type magnet 201. These MRI apparatuses 100 and200 are provided with a table 301 on which a subject 103 is placed. Thisembodiment is applicable to both the MRI apparatus 100 provided with thehorizontal magnetic field type magnet 101, and the MRI apparatus 200provided with the vertical magnetic field type magnet 201. Hereafter,the present invention will be explained for the MRI apparatus 100 havingthe horizontal magnetic field type magnet 101 as an example.

FIG. 2 is a block diagram showing the schematic configuration of the MRIapparatus 100. As shown in this diagram, the MRI apparatus 100 isprovided with the horizontal magnetic field type magnet 101, a gradientcoil 102 which generates a gradient magnetic field, a sequencer 104, anda transmit and receive RF coil 116 which generates a RF magnetic fieldand receives signals from the subject 103. The gradient coil 102 isconnected to a gradient coil power supply 105. The transmit and receiveRF coil 116 is connected to the RF magnetic field generator 106 and areceiver 108. The sequencer 104 sends commands to the gradient coilpower supply 105 and the RF magnetic field generator 106 to make themgenerate a gradient magnetic field and a RF magnetic field,respectively. The RF magnetic field is applied on the subject 103 viathe transmit and receive RF coil 116. RF signals generated from thesubject 103 upon application of the RF magnetic field are detected bythe transmit and receive RF coil 116, and detection is performed by thereceiver 108. A magnetic resonance frequency as the basis of thedetection in the receiver 108 is set by the sequencer 104. The detectedsignals are sent to a computer 109 via an A/D converter, and signalprocessing such as image reconstruction is performed in the computer.The results are displayed on a display 110. The detected signals andmeasurement conditions are stored in a storage medium 111 as required.The sequencer 104 controls the components so that each componentoperates according to timings and magnitudes programmed beforehand.Furthermore, when adjustment of uniformity of the static magnetic fieldis required, a shim coil 112 is used. In addition, as the aforementionedtransmit and receive RF coil 116, a transmit coil which generates a RFmagnetic field and a receive coil which receives signals from thesubject 103 may be separately provided, or one coil serving as the bothmay be used. In the following explanation of this embodiment, a case ofusing one coil serving as the both will be explained as an example.

The transmit and receive RF coil 116 according to this embodimentoperates as a double-tuned RF coil. FIG. 3 is a circuit diagram of adouble-tuned loop-type coil 150 used as the transmit and receive RF coil116 of this embodiment. In the diagram, the direction of the z axis inthe coordinate 9 is the static magnetic field direction. Thedouble-tuned loop-type coil 150 of this embodiment comprises a firstseries resonant circuit 41 in which eight inductors 19 and sevencapacitors 29 are alternately connected in series, a second seriesresonant circuit 42 in which a capacitor 22 (capacitance of thecapacitor 22 is represented as C_(B), henceforth the values arerepresented in the same manner) and an inductor 12 (L_(B)) are connectedin series, a third series resonant circuit 43 in which a capacitor 23(C_(C)) and an inductor 13 (L_(C)) are connected in series, and a signalprocessing circuit 45. The first series resonant circuit 41, the secondseries resonant circuit 42, and the third series resonant circuit 43 areconnected in parallel in this order, and the signal processing circuit45 is connected in parallel to the third series resonant circuit 43. Thedouble-tuned loop-type coil 150 of this embodiment is connected to theRF magnetic field generator 106 and the receiver 108 via the signalprocessing circuit 45. The signal processing circuit 45 includescircuits for processing signals such as balun circuit and impedancematching circuit, and may be further provided with a signal amplifier.

The inductor 19 represents one portion of inductor among portionsobtained by dividing the loop coil 1 made of a conductive material intoeight. For example, if inductance (L_(A)) of a typical loop coil shallbe 1 pH, the inductance of the inductor 19 is 125 nH. The capacitor 29represents one portion of capacitor among portions obtained by dividingcapacitor (C_(A)) inserted into the loop coil 1 in series into seven.The number of the divided portions may be changed depending on the valueof capacitance of the capacitor (C_(A)).

The resonance frequencies of the first series resonant circuit 41, thesecond series resonant circuit 42 and the third series resonant circuit43 are represented by f_(A), f_(B) and f_(C), respectively. Thedouble-tuned loop-type coil 150 of this embodiment is tuned to the twoof different nuclear magnetic resonance frequencies (first resonancefrequency f₁ and second resonance frequency f₂, f₁<f₂) of two kinds ofdifferent atomic nuclei by adjusting inductances and capacitances of theinductors and the capacitors. For this purpose, the resonancefrequencies f_(A) f_(B) and f_(C) of the series resonant circuits areadjusted so that they satisfy the condition represented by the followingEquation (1).f_(B)<f₁<f_(A)<f₂<f_(C)  (1)

Explained hereafter is that if the resonance frequencies of the seriesresonant circuits are adjusted so that they satisfy the condition ofEquation (1) mentioned above, two kinds of magnetic resonance signals offrequencies close to each other can be transmitted and received by thedouble-tuned loop-type coil 150 using practical inductances andcapacitances of the inductors and capacitors, which are constituents ofthe coil. As an example, one case will be explained here, where, amongthe magnetic resonance frequencies of two kinds of different atomicnuclei, the first magnetic resonance frequency f₁ is 120 MHz, thenuclear magnetic resonance frequency of fluorine in a static magneticfield strength of 3-T (tesla), and the second magnetic resonancefrequency f₂ is 128 MHz, the nuclear magnetic resonance frequency ofhydrogen nuclei in a static magnetic field strength of 3-T.

First, operation and characteristics of the double-tuned loop-type coil150 of this embodiment will be explained by using an equivalent circuitthereof. FIG. 4A, FIG. 4B and FIG. 4C are diagrams for explainingoperation of the double-tuned loop-type coil by using an equivalentcircuit. FIG. 4A shows an equivalent circuit 500 of the double-tunedloop-type coil 150 of this embodiment. In the circuit of this diagram,the inductance of the inductor 11 (L_(A)) is the combined value of theinductances of the eight inductors 19 connected in series in thedouble-tuned loop-type coil 150, and the capacitance of the capacitor 21(C_(A)) is the combined value of the capacitances of seven capacitors 29connected in series in the double-tuned loop-type coil 150,respectively. As shown in this diagram, the double-tuned loop-type coil150 of this embodiment can be represented by an equivalent circuit 500of a circuit, in which three of series resonant circuits (41′, 42, 43)each having an inductor and a capacitor, are connected in parallel.

Operation of a usual series resonant circuit 40 will be explained. FIG.5A and FIG. 5B are diagrams for explaining operation of the seriesresonant circuit 40. As shown in FIG. 5A, in the series resonant circuit40, an inductor 15 (L) and a capacitor 25 (C) are connected in series.When frequency of voltage to be applied is represented by f, and angularfrequency is represented by ω (ω=2 πf), impedance Z between the bothends of the series resonant circuit 40 is represented by Equation (2).

$\begin{matrix}{Z = {{{j\;\omega\; L} + \frac{1}{j\;\omega\; C}} = {{j\; 2\;\pi\;{fL}} + \frac{1}{j\; 2\;\pi\;{fC}}}}} & (2)\end{matrix}$

The impedance Z changes depending on the frequency f as shown in FIG.5B, and the circuit resonates at a frequency f=f_(R). In FIG. 5B, in theregion of frequency higher than the resonance frequency f_(R) of theseries resonant circuit 40 (f_(R)<f), the impedance Z is represented byEquation (3) and operates as inductive reactance.

$\begin{matrix}{Z = {j\; 2\;\pi\;{fL}\frac{\left( {f/f_{R}} \right)^{2} - 1}{\left( {f/f_{R}} \right)^{2}}}} & (3)\end{matrix}$Here, the value L′ of the apparent inductance of the series resonantcircuit 40 is represented by Equation (4).

$\begin{matrix}{L^{\prime} = {\frac{\left( {f/f_{R}} \right)^{2} - 1}{\left( {f/f_{R}} \right)^{2}}L}} & (4)\end{matrix}$On the other hand, in the region of frequency lower than the resonancefrequency f_(R) of the series resonant circuit 40 (f<f_(R)), theimpedance Z is represented by Equation (5), and operates as capacitivereactance.

$\begin{matrix}{Z = \frac{1 - \left( {f/f_{R}} \right)^{2}}{j\; 2\;\pi\;{fC}}} & (5)\end{matrix}$Here, the value C′ of the apparent capacitance of the series resonantcircuit 40 is represented by Equation (6).

$\begin{matrix}{C^{\prime} = \frac{C}{1 - \left( {f/f_{R}} \right)^{2}}} & (6)\end{matrix}$

As described above, the series resonant circuit 40 differently operatesaccording to the frequency of the applied voltage, i.e., differentlyoperates on the both sides of the resonance frequency as a border. Theresonance frequencies f_(A), f_(B) and f_(C) of the series resonantcircuits 41′, 42 and 43 of the equivalent circuit 500 of thedouble-tuned loop-type coil 150 of this embodiment are adjusted so as tosatisfy the condition of Equation (1). Therefore, when a RF signal ofthe first resonance frequency f₁ is applied, the series resonant circuit41′ and the series resonant circuit 43 of the equivalent circuit 500operate as capacitive reactance (capacitor), and the series resonantcircuit 42 operates as inductive reactance (inductor). The configurationof the equivalent circuit 500 in this case is shown in FIG. 4B. As shownin this diagram, when a RF signal of the first resonance frequency f₁ isapplied, the equivalent circuit 500 is represented as a parallelresonant circuit 501 in which a capacitor 76 (C_(A)′), an inductor 73(L_(B)′) and a capacitor 77 (C_(C)′) are connected in parallel.

On the other hand, when a RF signal of the second resonance frequency f₂is applied, the series resonant circuit 41′ and the series resonantcircuit 42 of the equivalent circuit 500 operate as inductive reactance(inductor), and the series resonant circuit 43 operates as capacitivereactance (capacitor). The configuration of the equivalent circuit 500in this case is shown in FIG. 4C. As shown in this diagram, when a RFsignal of the second resonance frequency f₂ is applied, the equivalentcircuit 500 is represented as a parallel resonant circuit 502 in whichan inductor 74 (L_(A)″), an inductor 75 (L_(B)″) and a capacitor 78(C_(C)″) are connected in parallel.

Therefore, if the capacitances and inductances of the capacitors andinductors are adjusted, respectively, so that the resonance frequency ofthe parallel resonant circuit 501 should be the first resonancefrequency f₁, and the resonance frequency of the parallel resonantcircuit 502 should be the second resonance frequency f₂, thedouble-tuned loop-type coil 150 of this embodiment represented by thisequivalent circuit 500 resonates at the first resonance frequency f₁ andsecond resonance frequency f₂. That is, it comes to be able to transmitand receive two kinds of magnetic resonance signals. Hereafter,adjustment of the capacitances and inductances of the capacitors andinductors will be explained.

The capacitances C_(A)′ and C_(C)′ of the capacitors 76 and 77 of theparallel resonant circuit 501 are represented by Equations (7) and (8)mentioned below in accordance with Equation (6). Moreover, theinductance L_(B)′ of the inductor 73 is represented by Equation (9)mentioned below in accordance with Equation (4).

$\begin{matrix}{C_{A}^{\prime} = \frac{C_{A}}{1 - \left( {f/f_{A}} \right)^{2}}} & (7) \\{C_{C}^{\prime} = \frac{C_{C}}{1 - \left( {f/f_{C}} \right)^{2}}} & (8) \\{L_{B}^{\prime} = {\frac{\left( {f/f_{B}} \right)^{2} - 1}{\left( {f/f_{B}} \right)^{2}}L_{B}}} & (9)\end{matrix}$

Here, resonance frequency f_(0p) of a parallel resonant circuitconstituted by an inductor and a capacitor, inductance L of the inductorand capacitance C of the capacitor generally satisfy the followingcondition.

$\begin{matrix}{f_{op} = \frac{1}{2\;\pi\sqrt{LC}}} & (10)\end{matrix}$

When the parallel resonant circuit 501 is adjusted so as to be tuned tothe first resonance frequency f₁, the resonance frequency of theparallel resonant circuit 501 shall be the first resonance frequency f₁.Therefore, f₁, capacitances C_(A)′ and C_(C)′ of the capacitors 76 and77, and inductance L_(B)′ of the inductor 73 satisfy Equation (10).Accordingly, the relation of f₁, C_(A)′, C_(C)′ and L_(B)′ isrepresented by Equation (11).

$\begin{matrix}{f_{1} = \frac{1}{2\;\pi\sqrt{L_{B}^{\prime}\left( {C_{A}^{\prime} + C_{C}^{\prime}} \right)}}} & (11)\end{matrix}$

When Equations (7), (8), (9) and (11) are solved for L_(A), L_(B) andL_(C), inductances of the inductors 11, 12 and 13 (L_(A), L_(B), L_(C))have the following relation.

$\begin{matrix}{{\frac{f_{1}^{2}}{f_{1}^{2} - f_{B}^{2}}\frac{1}{L_{B}}} = {{\frac{f_{1}^{2}}{f_{A}^{2} - f_{1}^{2}}\frac{1}{L_{A}}} + {\frac{f_{1}^{2}}{f_{C}^{2} - f_{1}^{2}}\frac{1}{L_{C}}}}} & (12)\end{matrix}$

When Equations (9), (7), (8) and (11) are similarly solved for C_(A),C_(B) and C_(C), capacitances of the capacitors 21, 22 and 23 (C_(A),C_(B), C_(C)) have the following relation.

$\begin{matrix}{\frac{C_{B}}{\left( {f_{1}/f_{B}} \right)^{2} - 1} = {\frac{C_{A}}{1 - \left( {f_{1}/f_{A}} \right)^{2}} + \frac{C_{C}}{1 - \left( {f_{1}/f_{C}} \right)^{2}}}} & (13)\end{matrix}$

On the other hand, inductances L_(A)″ and L_(B)″ of the inductors 74 and75 of the parallel resonant circuits 502 are represented by Equations(14) and (15) mentioned below, respectively, in accordance with Equation(4). Moreover, capacitance C_(C)″ of the capacitor 78 is represented byEquation (16) mentioned below in accordance with Equation (6).

$\begin{matrix}{L_{A}^{''} = {\frac{\left( {f/f_{A}} \right)^{2} - 1}{\left( {f/f_{A}} \right)^{2}}L_{A}}} & (14) \\{L_{B}^{''} = {\frac{\left( {f/f_{B}} \right)^{2} - 1}{\left( {f/f_{B}} \right)^{2}}L_{B}}} & (15) \\{C_{C}^{''} = \frac{C_{C}}{1 - \left( {f/f_{C}} \right)^{2}}} & (16)\end{matrix}$

When the parallel resonant circuit 502 is adjusted so as to be tuned tothe second resonance frequency f₂, the resonance frequency of theparallel resonant circuit 502 shall be the second resonance frequencyf₂. Therefore, f₂, inductances L_(A)″ and L_(B)″ of the inductors 74 and75, and capacitance C_(C)″ of the capacitor 78 satisfy the condition ofEquation (10). That is, the relation of f₂, L_(A)″, L_(B)″ and C_(C)″ isrepresented by Equation (17).

$\begin{matrix}{f_{2} = {\frac{1}{2\;\pi}\sqrt{\frac{L_{A}^{'' - 1} + L_{B}^{'' - 1}}{C_{C}^{''}}}}} & (17)\end{matrix}$

When Equations (14), (15), (16) and (17) are solved for L_(A), L_(B) andL_(C), the inductances of the inductors 11, 12 and 13 (L_(A), L_(B),L_(C)) satisfy the following relation.

$\begin{matrix}{{\frac{f_{2}^{2}}{f_{C}^{2} - f_{2}^{2}}\frac{1}{L_{C}}} = {{\frac{f_{2}^{2}}{f_{2}^{2} - f_{A}^{2}}\frac{1}{L_{A}}} + {\frac{f_{2}^{2}}{f_{2}^{2} - f_{B}^{2}}\frac{1}{L_{B}}}}} & (18)\end{matrix}$

When Equations (14), (15), (16) and (17) are similarly solved for C_(A),C_(B) and C_(C), capacitances of the capacitors 21, 22 and 23 (C_(A),C_(B), C_(C)) satisfy the following relation.

$\begin{matrix}{\frac{C_{C}}{1 - \left( {f_{2}/f_{C}} \right)^{2}} = {\frac{C_{A}}{\left( {f_{2}/f_{A}} \right)^{2} - 1} + \frac{C_{B}}{\left( {f_{2}/f_{B}} \right)^{2} - 1}}} & (19)\end{matrix}$

Therefore, since the inductances L_(A), L_(B) and L_(C) of the inductorsneed to simultaneously satisfy the relations of Equations (12) and (18),the inductance of the inductor 12 (L_(B)) and the inductance of theinductor 13 (L_(C)) are represented by Equations (21) and (20),respectively, by using the resonance frequencies f₁, f₂, f_(A), f_(B),f_(C) and L_(A).

$\begin{matrix}{L_{B} = {\left( {\frac{f_{C}^{2} - f_{1}^{2}}{f_{1}^{2} - f_{B}^{2}} - \frac{f_{C}^{2} - f_{2}^{2}}{f_{2}^{2} - f_{B}^{2}}} \right)\left( {\frac{f_{C}^{2} - f_{2}^{2}}{f_{2}^{2} - f_{A}^{2}} + \frac{f_{C}^{2} - f_{1}^{2}}{f_{A}^{2} - f_{1}^{2}}} \right)^{- 1}L_{A}}} & (20) \\{L_{C} = {\left( {\frac{f_{2}^{2} - f_{B}^{2}}{f_{C}^{2} - f_{2}^{2}} - \frac{f_{1}^{2} - f_{B}^{2}}{f_{C}^{2} - f_{1}^{2}}} \right)\left( {\frac{f_{1}^{2} - f_{B}^{2}}{f_{A}^{2} - f_{1}^{2}} + \frac{f_{2}^{2} - f_{B}^{2}}{f_{2}^{2} - f_{A}^{2}}} \right)^{- 1}L_{A}}} & (21)\end{matrix}$

On the other hand, since the capacitances C_(A), C_(B) and C_(C) of thecapacitors need to simultaneously satisfy the relations of Equations(13) and (19), the capacitances of the capacitor 22 (C_(B)) and thecapacitor 23 (C_(C)) are represented by Equations (22) and (23),respectively, by using the resonance frequencies f₁, f₂, f_(A), f_(B),f_(C) and C_(A).

$\begin{matrix}{C_{B} = {\left( {\frac{1 - \left( {f_{1}/f_{C}} \right)^{2}}{1 - \left( {f_{1}/f_{A}} \right)^{2}} + \frac{1 - \left( {f_{2}/f_{C}} \right)^{2}}{\left( {f_{2}/f_{A}} \right)^{2} - 1}} \right)\left( {\frac{1 - \left( {f_{1}/f_{C}} \right)^{2}}{\left( {f_{1}/f_{B}} \right)^{2} - 1} - \frac{1 - \left( {f_{2}/f_{C}} \right)^{2}}{\left( {f_{2}/f_{B}} \right)^{2} - 1}} \right)^{- 1}C_{A}}} & (22) \\{C_{C} = {\left( {\frac{\left( {f_{1}/f_{B}} \right)^{2} - 1}{1 - \left( {f_{1}/f_{A}} \right)^{2}} + \frac{\left( {f_{2}/f_{B}} \right)^{2} - 1}{\left( {f_{2}/f_{A}} \right)^{2} - 1}} \right)\left( {\frac{\left( {f_{2}/f_{B}} \right)^{2} - 1}{1 - \left( {f_{2}/f_{C}} \right)^{2}} - \frac{\left( {f_{1}/f_{B}} \right)^{2} - 1}{1 - \left( {f_{1}/f_{C}} \right)^{2}}} \right)^{- 1}C_{A}}} & (23)\end{matrix}$

By using the aforementioned relations obtained for the equivalentcircuit 500, capacitances and inductances of the capacitors andinductors of the double-tuned loop-type coil 150 of this embodiment arecalculated.

First, the resonance frequency f_(A) of the series resonant circuit 41is determined. As mentioned above, the first resonance frequency f₁ is120 MHz, and the second resonance frequency f₂ is 128 MHz in this case.Therefore, the resonance frequency f_(A) of the series resonant circuit41 is defined to be between 120 MHz and 128 MHz in accordance withEquation (1). Here, for example, it is determined to be 124 MHz.

Then, the combined value of the inductances of the series resonantcircuit 41, L_(A), is determined. In this case, it is determined to be,for example, 1 μH, which is an inductance of a typical loop coil.Therefore, the inductance of each of the eight inductors 19 constitutingthe series resonant circuit 41 should be 125 nH, respectively.

Further, the combined value of the capacitances of the series resonantcircuit 41, C_(A), is determined. In a series resonant circuit, theresonance frequency f_(os), inductance L and capacitance C thereof aregenerally in the relation represented by Equation (24) mentioned below.

$\begin{matrix}{f_{os} = \frac{1}{2\;\pi\sqrt{LC}}} & (24)\end{matrix}$If this relation is applied to the series resonant circuit 41, thecombined value of capacitances, C_(A), is calculated to be 1.65 pF.Therefore, the capacitance of each of the seven capacitors 29constituting the series resonant circuit 41 should be 11.6 pF.

Then, the resonance frequency f_(B) of the series resonant circuit 42and the resonance frequency f_(C) of the series resonant circuit 43 aredetermined so as to satisfy Equation (1). In this determination, thevalues calculated in accordance with Equations (20), (21), (22) and (23)are determined so that the inductance should be between 10 and 200 nH,and the capacitance should be 10 to 200 pF, in order that the RF loss ofthe inductors and capacitors constituting the respective circuits shouldbecome low, and adjustment thereof should become easy. A frequencyregion of f_(B) and f_(C) satisfying Equation (1) is shown in FIG. 6A.Further, frequency regions of such f_(B) and f_(C) that L_(B) and L_(C)calculated in accordance with Equations (20) and (21) should be between10 and 200 nH, which enable actual production and adjustment thereof,are shown in FIG. 6B and FIG. 6C. Similarly, frequency regions of suchf_(B) and f_(C) that C_(B) and C_(C) calculated in accordance withEquations (22) and (23) should be between 10 and 200 pF, which providerelatively low RF loss, are shown in FIG. 6D and FIG. 6E. Moreover,frequency regions of f_(B) and f_(C) that satisfy all the conditions ofthe regions of FIG. 6A to FIG. 6E are shown in FIG. 6F. Any combinationof f_(B) and f_(C) in this region is sufficient. In this case, forexample f_(B) and f_(C) are determined to be 93.6 MHz and 156 MHz,respectively.

Finally, by using the resonance frequencies f₁, f₂, f_(B) and f_(C), andL_(A) and C_(A) determined as described above, L_(B), L_(C), C_(B) andC_(C) are calculated in accordance with Equations (20), (21), (22) and(23). As a result, L_(B), L_(C), C_(B) and C_(C) are determined to be24.5 nH, 30.7 nH, 42.6 pF and 94.3 pF, respectively. These are withinthe aforementioned ranges of inductance and capacitance.

By adjusting the inductances and capacitances as follows: L_(A)=1 μH,L_(B)=24.5 nH, L_(C)=30.7 nH, C_(A)=1.65 pF, C_(B)=42.6 pF andC_(C)=94.3 pF as described above, the double-tuned loop-type coil 150 ofthis embodiment resonates at both the frequencies of 120 MHz, thenuclear magnetic resonance frequency of fluorine at 3-T, and 128 MHz,the nuclear magnetic resonance frequency of hydrogen nuclei at 3-T, andit transmits and receives magnetic resonance signals of fluorine andhydrogen nuclei.

As described above, the double-tuned loop-type coil 150 of thisembodiment is tuned to two kinds of magnetic resonance frequencies closeto each other so as to irradiate RF magnetic fields of two kinds ofmagnetic resonance frequencies highly efficiently and uniformly andreceive two kinds of magnetic resonance signals with high sensitivityand uniform sensitivity profile. Further, it realizes transmission andreception of magnetic resonance signals of two kinds of frequenciesclose to each other by using capacitors and inductors having practicalcapacitance or inductance without using a capacitor having a largecapacitance inviting RF loss or an inductor having a small inductanceinviting difficulty in adjustment. Therefore, RF loss caused by theinductor or capacitor can be reduced, and reception sensitivity andtransmission efficiency of RF coils for magnetic resonance signals oftwo kinds of frequencies close to each other can be improved.

Further, as seen from the aforementioned configuration, the double-tunedloop-type coil 150 of this embodiment does not have any trap circuit,which is not involved in signal detection of the RF coil, in the signaldetection coil. Therefore, uniformity of the sensitivity profile of theRF coil can be improved without disturbance of sensitivity profile ofthe RF coil by trap circuit.

Furthermore, by disposing the loop coil 1 of the double-tuned loop-typecoil 150 of this embodiment in contact with the subject 103, magneticresonance signals emitted from portions around the portion in contactwith the loop coil can be detected with high sensitivity.

Explained for this embodiment as an example is a case where thecombination of the first resonance frequency and the second resonancefrequency are that of the nuclear magnetic resonance frequencies offluorine and hydrogen nuclei. However, the combination is not limited tothis combination, provided that a combination in which one resonancefrequency is not more than 70% of the other resonance frequency ispreferred. For example, combinations of fluorine and helium (³He),phosphorus (³¹P) and lithium (⁷Li), xenon (¹²⁹Xe) and sodium (²³Na),xenon (¹²⁹Xe) and carbon (¹³C), sodium (²³Na) and carbon (¹³C), oxygen(¹⁹O) and heavy water (¹H), and so forth can be contemplated. However,the combinations of atomic nuclei are of course not limited to these.

In addition, the shape of the double-tuned loop-type coil 150 of thisembodiment is not limited to the aforementioned shape. It is sufficientthat equivalent circuit thereof is equivalent to the equivalent circuit500.

For example, the portion of the loop coil 1 may have a shape ofsaddle-type coil. A double-tuned saddle-type coil 151 is shown in FIG.7, which is a modified version of the double-tuned loop-type coil ofthis embodiment. In the diagram, the direction of the z axis in thecoordinate 9 is the static magnetic field direction. As shown in thediagram, the double-tuned saddle-type coil 151 comprises two facingloops of the saddle-type loop coil 1 connected so that they shouldgenerate magnetic fields in the same direction, and it has a shape inwhich the planes of the loops are curved so that planes should be alonga cylindrical side.

Further, for example, the portion of the loop coil 1 may have a shape ofbutterfly-type coil. A double-tuned butterfly-type coil 152 is shown inFIG. 8, which is a modified version of the double-tuned loop-type coilof this embodiment. In the diagram, the direction of the z axis in thecoordinate 9 is the static magnetic field direction. As shown in thediagram, the double-tuned saddle-type coil 152 has a shape in which twoadjacent loops in the same plane of the butterfly-shaped loop coil 1 areconnected so that they should generate magnetic fields in reversedirections.

Further, for example, the portion of the loop coil 1 may have a shape ofsolenoid-type coil. A double-tuned solenoid-type coil 153 is shown inFIG. 9, which is a modified version of the double-tuned loop-type coilof this embodiment. In the diagram, the direction of the z axis in thecoordinate 9 is the static magnetic field direction.

Since the double-tuned saddle-type coil 151, the double-tunedbutterfly-type coil 152, and the double-tuned solenoid-type coil 153 arerepresented by the equivalent circuit 500, the circuit configuration andoperation principle thereof are the same as those of the double-tunedloop-type coil 150. Therefore, the double-tuned saddle-type coil 151,the double-tuned butterfly-type coil 152, and the double-tunedsolenoid-type coil 153 operate as RF coils for magnetic resonancesignals having frequencies close to each other, of which typicalexamples are signals of a combination of hydrogen nuclei and fluorinenuclei. However, since the coil shapes of them are different from thatof the aforementioned double-tuned loop-type coil 150, and inductance ofthe inductor 11 (L_(A)) and capacitance of the capacitor 21 (C_(A)) ofthe loop coil vary, L_(B), L_(C), C_(B) and C_(C) should becorrespondingly determined.

As described above, the double-tuned saddle-type coil 151, thedouble-tuned butterfly-type coil 152, and the double-tuned solenoid-typecoil 153 enable to realize an RF coil which can transmit and receivemagnetic resonance signals of two kinds of frequencies close to eachother without using a capacitor having such a large capacitance that RFloss should be accompanied and an inductor having a small inductancewhich causes difficulty in adjustment of inductance. Therefore, the RFloss caused by an inductor or a capacitor can be markedly decreased, andreception sensitivity and transmission efficiency of an RF coil formagnetic resonance signals of two kinds of frequencies close to eachother are improved. Further, since any trap circuit which does notparticipate in signal detection of the RF coil is not disposed in thesignal detection coil, sensitivity profile of the RF coil is notdisturbed by trap circuit, and thus uniformity of the sensitivityprofile of the RF coil can be improved.

Furthermore, since the double-tuned saddle-type coil 151 has a saddleshape, by placing the subject 103 such as arm, leg or body thereof inthe saddle-type coil as shown in FIG. 7, two kinds of magnetic resonancesignals from regions locating in deep parts can be detected with highsensitivity and uniform distribution, in addition to signals from thesurface of the subject 103.

Further, since the coil of double-tuned butterfly-type coil 152 has abutterfly shape, the subject 103 such as arm, leg or body thereof doesnot enter into a closed space. By disposing the subject 103 under orover the butterfly-type coil as shown in FIG. 8, two kinds of magneticresonance signals from regions locating in deep parts can be detectedwith high sensitivity and uniform distribution.

Further, since the double-tuned solenoid-type coil 153 has a solenoidshape, by placing the subject 103 such as arm, leg or body thereof inthe solenoid-type coil as shown in FIG. 9, two kinds of magneticresonance signals from regions locating in deep parts can be detectedwith high sensitivity and uniform distribution, in addition to signalsfrom the surface of the subject 103. Moreover, the solenoid-type coilshows a uniform sensitivity profile in a larger region compared with thesaddle-type coil.

In these modified versions of this embodiment, one capacitor 21 isdisposed in the loop coil 1. However, two or more capacitors may also bedisposed in the loop coil.

Further, in the aforementioned embodiment and the modified versionsthereof, the double-tuned RF coil which can transmit and receive twokinds of different magnetic resonance signals is explained as anexample. However, magnetic resonance signals which can be transmittedand received by the RF coil to which the present invention is applicableare not limited to two kinds of magnetic resonance signals. For example,the coil may be a triple-tuned loop-type coil which can transmit andreceive three kinds of different magnetic resonance signals.

A triple-tuned loop-type coil 154 as a modified version of thedouble-tuned loop-type coil of this embodiment is shown in FIG. 10. Asshown in this diagram, the triple-tuned loop-type coil 154 comprises afourth series resonant circuit 44 including a capacitor 24 (C_(D)) andan inductor 14 (L_(D)) connected in series, which is connected inparallel with the third series resonant circuit 43, in addition to thecomponents of the double-tuned loop-type coil 150.

The resonance frequencies (f_(A), f_(B), f_(C), f_(D)) of the first,second, third and fourth series resonant circuits 41, 42, 43 and 44 areadjusted so as to satisfy the relation (Equations 25) mentioned below sothat the triple-tuned loop-type coil 154 should resonate at the first,second and third resonance frequencies (f₁, f₂, f₃) corresponding to themagnetic resonance frequencies of the first, second and third elements.f_(B)<f₁<f_(A)<f₂<f_(C)<f₃<f_(D)  (25)

Also in this modification, the inductances L_(B), L_(C) and L_(D) of theinductors 12, 13 and 14 and the capacitances C_(B), C_(C) and C_(D) ofthe capacitors 22, 23 and 24 are determined according to the first,second and third resonance frequencies f₁, f₂ and f₃, the combined valueof inductance L_(A) of the inductors and the combined value ofcapacitance C_(A) of the capacitors in the loop coil 1 portion as in thecase of the double-tuned loop-type coil 150.

Operation and characteristics of the triple-tuned loop-type coil 154 ofthis modification will be explained by using an equivalent circuit 600shown in FIG. 11A. As shown in this diagram, the triple-tuned loop-typecoil 154 of this modification is represented by the equivalent circuit600 in which four series resonant circuits 41′, 42, 43 and 44, eachhaving inductors and capacitors, are connected in parallel. Theresonance frequencies of the series resonant circuits 41′, 42, 43 and 44are f_(A), f_(B), f_(C), and f_(D), respectively.

Since the equivalent circuit 600 is adjusted so that the relation ofEquation (25) should be satisfied, if a RF signal of the first resonancefrequency f₁ is applied, the second series resonant circuit 42 operatesas inductive reactance and can be regarded as an inductor 84 (L_(B)′).The first series resonant circuit 41′, the third series resonant circuit43 and the fourth series resonant circuit 44 operate as capacitivereactances, and can be regarded as capacitors 94, 95 and 96 (C_(A)′,C_(C)′, C_(D)′), respectively.

Therefore, at the first resonance frequency f₁, the equivalent circuit600 is represented as a parallel resonant circuit 601 shown in FIG. 11B,in which the inductor 84, capacitors 94, 95 and 96 are connected inparallel. If the resonance frequency of the parallel resonant circuit601 is adjusted to the first resonance frequency f₁ in this case, theequivalent circuit 600, i.e., the triple-tuned loop-type coil 154,resonates at the first resonance frequency f₁. Further, the relation ofthe resonance frequency f₁, capacitances C_(A)′, C_(C)′ and C_(D)′ ofthe capacitors 94, 95 and 96, and inductance L_(B)′ of the inductor 84is represented as follows according to Equation (10).

$\begin{matrix}{f_{1} = \frac{1}{2\;\pi\sqrt{L_{B}^{\prime}\left( {C_{A}^{\prime} + C_{C}^{\prime} + C_{D}^{\prime}} \right)}}} & (26)\end{matrix}$

Further, since the equivalent circuit 600 is adjusted so to satisfy therelation of Equation (25), if a RF signal of the second resonancefrequency f₂ is applied, the first and second series resonant circuits41′ and 42 operate as inductive reactances and can be regarded asinductors 85 and 86 (L_(A)″, L_(B)″). The third and fourth seriesresonant circuits 43 and 44 operate as capacitive reactances, and can beregarded as capacitors 97 and 98 (C_(C)″, C_(D)″).

Therefore, at the second resonance frequency f₂, the equivalent circuit600 is represented as a parallel resonant circuit 602 shown in FIG. 11C,in which the inductors 85 and 86 and capacitors 97 and 98 are connectedin parallel. If the resonance frequency of the parallel resonant circuit602 is adjusted to the second resonance frequency f₂ in this case, theequivalent circuit 600, i.e., the triple-tuned loop-type coil 154,resonates at the second resonance frequency f₂. Further, the relation ofthe second resonance frequency f₂, inductances L_(A)″ and L_(B)″ of theinductors 85 and 86, and capacitances C_(B)″ and C_(D)″ of thecapacitors 97 and 98 constituting the parallel resonant circuit 602 isrepresented as follows according to Equation (10).

$\begin{matrix}{f_{2} = {\frac{1}{2\;\pi}\sqrt{\frac{L_{A}^{''} + L_{B}^{''}}{L_{A}^{''}{L_{B}^{''}\left( {C_{C}^{''} + C_{D}^{''}} \right)}}}}} & (27)\end{matrix}$

Further, since the equivalent circuit 600 is adjusted so as to satisfythe relation of Equation (25), if a RF signal of the third resonancefrequency f₃ is applied, the first, second and third series resonantcircuits 41′, 42 and 43 operate as inductive reactances and can beregarded as inductors 87, 88 and 89 (L_(A)′″, L_(B)′″, L_(C)′″).Similarly, the fourth series resonant circuits 44 operates as capacitivereactance at the third resonance frequency f₃, and can be regarded ascapacitor 99 (C_(D)′″).

Therefore, at the third resonance frequency f₃, the equivalent circuit600 is represented as a parallel resonant circuit 603 shown in FIG. 11D,in which the inductors 87, 88 and 89 and capacitor 99 are connected inparallel. If the resonance frequency of the parallel resonant circuit603 is adjusted to the third resonance frequency f₃ in this case, theequivalent circuit 600, i.e., the triple-tuned loop-type coil 154,resonates at the third resonance frequency f₃. Further, the relation ofthe third resonance frequency f₃, inductances L_(A)′″, L_(B)′″ andL_(C)′″ of the inductors 87, 88 and 89, and capacitance C_(D)′″ of thecapacitor 99 is represented as follows according to Equation (10).

$\begin{matrix}{f_{3} = {\frac{1}{2\;\pi}\sqrt{\frac{{L_{A}^{\prime\prime\prime}L_{B}^{\prime\prime\prime}} + {L_{B}^{\prime\prime\prime}L_{C}^{\prime\prime\prime}} + {L_{A}^{\prime\prime\prime}L_{C}^{\prime\prime\prime}}}{L_{A}^{\prime\prime\prime}L_{B}^{\prime\prime\prime}L_{C}^{\prime\prime\prime}C_{D}^{\prime\prime\prime}}}}} & (28)\end{matrix}$

When Equations (26), (27) and (28) are solved for L_(B), L_(C), L_(D),C_(B), C_(C) and C_(D), each of L_(B), L_(C), L_(D), C_(B), C_(C) andC_(D) is represented as a function including f_(B), f_(C) and f_(D) asvariables. By adjusting f_(B), f_(C) and f_(D) so that they shouldsatisfy the relation of Equation (25) and adjusting the inductances andcapacitances so that they should be within the ranges giving low RF lossand enabling easy adjustment, i.e., 10 nH<(L_(B), L_(C), L_(D))<200 nHand 10 pF<(C_(B), C_(C), C_(D))<200 pF, the triple-tuned loop-type coil154 resonates at the magnetic resonance frequencies of three kinds ofelements (f₁, f₂, f₃), and can transmit and receive magnetic resonancesignals.

As described above, the triple-tuned loop-type coil 154 can realize anRF coil which can transmit and receive magnetic resonance signals ofthree kinds of frequencies close to each other without using a capacitorhaving such a large capacitance that it should be accompanied by RF lossor an inductor having a small inductance causing difficulty inadjustment. Therefore, RF loss caused by inductor or capacitor can bemarkedly reduced, and reception sensitivity and transmission efficiencyof an RF coil for magnetic resonance signals of three kinds offrequencies close to each other can be improved. Further, since any trapcircuit which does not participate in signal detection of the RF coil isnot included in the signal detection coil, uniformity of the sensitivityprofile of the RF coil can be improved without disturbance ofsensitivity profile of the RF coil by trap circuit. Furthermore, bydisposing the loop coil 1 in contact with a subject 103, three kinds ofmagnetic resonance signals of which frequencies are close to one anotherfrom a portion near a contacting portion can be detected with highsensitivity.

In the above explanation, explained as an example is a case where theresonance frequencies of the first, second, third and fourth seriesresonant circuit 41, 42, 43 and 44 (f_(A), f_(B), f_(C), f_(D)) areadjusted to satisfy the relation of Equation (25) so that thetriple-tuned loop-type coil 154 should resonate at the first, second andthird resonance frequencies (f₁, f₂, f₃) corresponding to the magneticresonance frequencies of the first, second and third elements. However,the relation of the resonance frequencies of the series resonantcircuits 41, 42 and 43 and the resonance frequencies of triple-tunedloop-type coil 154 is not limited to this. For example, the relation maybe f_(B)<f₁<f_(C)<f₂<f_(A)<f₃<f_(D).

Further, the loop coil may be quadruple-tuned by further connecting aseries resonant circuit, in which a capacitor and an inductor areconnected in series, with the fourth series resonant circuit 44 inparallel. Furthermore, higher order tuning is also theoreticallypossible.

Second Embodiment

Hereafter, the second embodiment of the present invention will beexplained. The configuration of the MRI apparatus based on thisembodiment is basically the same as that of the first embodiment. Inthis embodiment, as the transmit and receive RF coil 116, a combinationof two of double-tuned loop-type coils 150 of the first embodiment isused to realize the quadrature detection (QD) for improving irradiationefficiency and reception sensitivity of the transmit and receive RFcoil. The configuration of this embodiment different from that of thefirst embodiment will be explained below.

FIG. 12A and FIG. 12B are diagrams for explaining the transmit andreceive RF coil 116 of this embodiment, and FIG. 12A is a circuitdiagram of the transmit and receive RF coil 116. In the diagram, thedirection of the z axis in the coordinate 9 is the static magnetic fielddirection. As shown in this diagram, the transmit and receive RF coil116 of this embodiment is provided with a first double-tuned loop-typecoil 61 and a second double-tuned loop-type coil 62. The configurationof each of the double-tuned loop-type coils 61 and 62 is the same asthat of the double-tuned loop-type coil 150 of the first embodiment.Further, the first double-tuned loop-type coil 61 and the seconddouble-tuned loop-type coil 62 of this embodiment are each adjusted soas to resonate at the first resonance frequency f₁ and second resonancefrequency f₂ (>f₁) like the double-tuned loop-type coil 150 of the firstembodiment.

The first double-tuned loop-type coil 61 and the second double-tunedloop-type coil 62 of the transmit and receive RF coil 116 of thisembodiment are disposed so that the loop planes 171 and 172 of the loopcoil portions 1 and 2 should be parallel to the z axis. Further, thesecond double-tuned loop-type coil 62 is disposed at a position definedby rotating the first double-tuned loop-type coil 61 by 90 degreesaround the z axis as a rotation axis.

FIG. 12B is a view of the transmit and receive RF coil 116 from thestatic magnetic field direction (z axis direction in the diagram). Asshown in this diagram, in the transmit and receive RF coil 116 of thisembodiment, the direction 63 of the magnetic field generated by thefirst double-tuned loop-type coil 61 and the direction 64 of themagnetic field generated by the second double-tuned loop-type coil 62are perpendicular to each other. Therefore, the first double-tunedloop-type coil 61 and the second double-tuned loop-type coil 62 are notmagnetically coupled, and they independently operate as RF coils for twokinds of magnetic resonance signals.

FIG. 13 is a block diagram for explaining connection of the firstdouble-tuned loop-type coil 61 and the second double-tuned loop-typecoil 62 of the transmit and receive RF coil 116 of this embodiment, a RFmagnetic field generator 106 and a receiver 108. Output of the RFmagnetic field generator 106 is inputted into a power divider 50 anddivided into two. At this time, the distribution is performed so thatthe phases of two of the divided outputs should be perpendicular to eachother. The outputs are inputted into a port 5 of the first double-tunedloop-type coil 61 and a port 6 of the second double-tuned loop-type coil62 via baluns 46, respectively. Further, the outputs from twodouble-tuned loop-type coils 61 and 62 are inputted into the signalamplifiers 47 via baluns 46, and the outputs of the signal amplifiers 47are inputted into a compositor (QD hybrid) 49 via phase shifters 48.Output of the compositor 49 is inputted into a receiver 108.

Operation of the transmit and receive RF coil 116 of this embodimentwill be explained below. If a RF signal of the first resonance frequencyf₁ or the second resonance frequency f₂ is transmitted by the RFmagnetic field generator 106, the signal is distributed by the powerdivider 50 into two so that the phases of the signals should beperpendicular to each other, and the signals are applied to the port 5and the port 6 via the baluns 46, respectively. The first double-tunedloop-type coil 61 and the second double-tuned loop-type coil 62 areadjusted so as to resonate at the first resonance frequency f₁ andsecond resonance frequency f₂, respectively, and therefore theyirradiate the subject 103 with RF signals of the first resonancefrequency f₁ or the second resonance frequency f₂ as a RF magneticfield. Since the phases of the RF magnetic fields irradiated by thefirst double-tuned loop-type coil 61 and the second double-tunedloop-type coil 62 are perpendicular to each other in this case, arotating magnetic field is generated in the subject 103 around the zaxis of the coordinates 9 as the revolution axis. As described above,the transmit and receive RF coil 116 of this embodiment realizestransmission of QD type.

Further, the first double-tuned loop-type coil 61 and the seconddouble-tuned loop-type coil 62 each detect perpendicular signalcomponents from the magnetic resonance signals of the first resonancefrequency f₁ or the second resonance frequency f₂ generated from thesubject 103. The detected signal components are amplified by the signalamplifiers 47, processed in the phase shifters 48, respectively, thensynthesized by the compositor 49 and sent to the receiver 108. Asdescribed above, the transmit and receive RF coil 116 of this embodimentrealizes reception of QD type.

As explained above, since the transmit and receive RF coil of thisembodiment realizes QD, it can highly efficiently irradiate the subject103 with a RF magnetic field, and can detect two kinds of magneticresonance signals with higher sensitivity, in addition to the effectbrought by the double-tuned loop-type coil 150 of the first embodiment.

This embodiment is explained above by exemplifying a case where two ofthe double-tuned loop-type coils 150 of the first embodiment arecombined in order to realize QD. However, coils to be combined in orderto realize QD are not limited to these. Any combination of two coilswhich can be disposed so that the magnetic fields generated by the coilsshould be perpendicular to each other may be used. For example, two ofsaddle-type coils may be disposed so that one of them should berotationally shifted by 90 degrees from the other around the z axis asthe rotation axis, or a solenoid-type coil and a saddle-type coil may bedisposed so that directions of the cylinders formed by them should bethe same.

Third Embodiment

Hereafter, the third embodiment of the present invention will beexplained. The configuration of the MRI apparatus of this embodiment isbasically the same as that of the first embodiment. In this embodiment,a transmit RF coil 116 a and a receive RF coil 116 b are independentlyprovided instead of the transmit and receive RF coil 116 of the firstembodiment. Here, explanation will be made by exemplifying a case wherea double-tuned birdcage RF coil having a birdcage shape is used as thetransmit RF coil 116 a, and a double-tuned loop-type coil having a loopcoil shape is used for the receive RF coil 116 b. Hereafter, theembodiment will be explained by emphasizing the configuration differentfrom that of the first embodiment.

FIG. 14 is a block diagram for explaining connection of the RF coil ofthis embodiment, a RF magnetic field generator 106 and a receiver 108.As shown in this diagram, the RF coil of this embodiment is providedwith a double-tuned birdcage RF coil 70, which is the transmit RF coil116 a, a double-tuned loop-type RF coil 71, which is a receive RF coil116 b, magnetic coupling preventing circuits 54 and 68 for preventingmagnetic coupling of the double-tuned birdcage RF coil 70 and thedouble-tuned loop-type coil 71, a magnetic coupling preventing circuitdriver 115 for driving the magnetic coupling preventing circuits 54 and68, and a power divider 50 which divides output of the RF magnetic fieldgenerator 106. In this embodiment, the magnetic coupling preventingcircuit 54 is serially inserted into the loop portion of thedouble-tuned birdcage RF coil 70, and the magnetic coupling preventingcircuit 68 is serially inserted into the double-tuned loop-type coil 71.

The double-tuned loop-type coil 71 of this embodiment is constituted sothat the equivalent circuit thereof should be equivalent to theequivalent circuit 500 of the first embodiment, and it is adjusted so asto resonate at the first resonance frequency f₁ and second resonancefrequency f₂. The double-tuned birdcage RF coil 70 is constituted with acircuit of conventional type, and is adjusted so as to resonate at thefirst resonance frequency f₁ and the second resonance frequency f₂.

Output of the RF magnetic field generator 106, which generates a RFmagnetic field of the first resonance frequency f₁, is inputted into apower divider 50 and is divided into two, and the outputs are inputtedinto pickup coils 65 via baluns 46. Output of RF magnetic fieldgenerator 106, which generates a RF magnetic field having the secondresonance frequency f₂, is inputted into the power divider 50 anddivided into two, and the outputs are inputted into pickup coils 66 viabaluns 46. The pickup coils 65 and 66 are disposed so as to transmit RFsignals of the first resonance frequency f₁ and the second resonancefrequency f₂ to the double-tuned birdcage RF coil 70, respectively. Thedouble-tuned birdcage RF coil 70 is provided with two or more magneticcoupling preventing circuits 54. Two or more signal wires for control 51are connected from a magnetic coupling preventing circuit driver 115 tothe magnetic coupling preventing circuits 54. The double-tuned loop-typecoil 71 is disposed inside of the double-tuned birdcage RF coil 70 so asto be disposed at a position close to the subject 103. Output of thedouble-tuned loop-type coil 71 is inputted into the signal amplifier 47via the baluns 46, and inputted into the receiver 108 from the signalamplifier 47. Two or more signal wires for control 51 are connected fromthe magnetic coupling preventing circuit driver 115 to the magneticcoupling preventing circuit 68.

FIG. 15A is a diagram for explaining the configuration and arrangementof the double-tuned birdcage RF coil 70 of this embodiment. As shown inthis diagram, the double-tuned birdcage RF coil 70 of this embodiment isprovided with two loop conductive materials 8 disposed so that both loopplanes are faced each other having a common axis perpendicular to theloop planes, and connected with two or more linear conductive materials7 (eight conductive materials are used in FIG. 15A as an example)parallel to the direction of the axis perpendicular to the loop planesof the loop conductive materials 8. Two or more magnetic couplingpreventing circuits 54 are inserted into each loop conductive material8. Further, the double-tuned birdcage RF coil 70 is disposed so that thecentral axis of the cylinder shape formed by it should be parallel tothe z axis direction.

FIG. 15B is a diagram for explaining the configuration of the magneticcoupling preventing circuit 54 of this embodiment. As shown in thisdiagram, in the magnetic coupling preventing circuit 54 of thisembodiment, an inductor 18 and a series circuit having a capacitor 26and another capacitor 27 connected in series are connected in parallel.A circuit having a PIN diode 30 and an inductor 16 connected in seriesis connected to the capacitor 26 in parallel, and a circuit having a PINdiode 31 and an inductor 17 connected in series is connected to thecapacitor 27 in parallel. The PIN diodes have a characteristic that theybecomes substantially conductive when a direct current higher than acertain level flows along the forward direction of the diodes, andturning-on and -off of the diodes can be controlled with a directcurrent. Output terminals of the magnetic coupling preventing circuitdriver 115 are connected to the junction point of the PIN diode 30 andthe inductor 16 and the junction point of the PIN diode 31 and theinductor 17. By controlling tuning-on and -off of the PIN diodes 30 and31 of the magnetic coupling preventing circuit 54 with a control current51 from the magnetic coupling preventing circuit driver 115, thedouble-tuned birdcage coil 70 is operated as the transmit RF coil 116 aat the time of RF signal transmission, and the double-tuned birdcagecoil 70 is made to have high impedance to prevent interference with thereceive RF coil 116 b (double-tuned loop-type coil 71) at the time of RFsignal reception. The details of this operation will be described later.

FIG. 16A is a diagram for explaining the configuration of thedouble-tuned loop-type coil 71. In the diagram, the direction of the zaxis in the coordinate 9 corresponds to the static magnetic fielddirection. The double-tuned loop-type coil 71 of this embodiment isbasically the same as the double-tuned loop-type coil 150 of the firstembodiment, and is further provided with a magnetic coupling preventingcircuit 68 in the loop coil 1. In the magnetic coupling preventingcircuit 68 shown in FIG. 16B, a capacitor 26 and another capacitor 27are connected in series. A circuit having a PIN diode 30 and an inductor16 connected in series is connected to the capacitor 26 in parallel, anda circuit having a PIN diode 31 and an inductor 17 connected in seriesis connected to the capacitor 27 in parallel. The PIN diodes have acharacteristic that they becomes substantially conductive when a directcurrent higher than a certain level flows along the forward direction ofthe diodes, and on/off of the diodes can be controlled with a directcurrent. Output terminals of the magnetic coupling preventing circuitdriver 115 are connected to the junction point of the PIN diode 30 andthe inductor 16 and the junction point of the PIN diode 31 and theinductor 17. On/off is controlled with a control current 51 from themagnetic coupling preventing circuit driver 115, so that thedouble-tuned loop-type coil 71 should operate as the receive RF coil 116b at the time of RF signal reception, and the double-tuned loop-typecoil 71 should be made to have high impedance to prevent interferencewith the transmit RF coil 116 a (double-tuned birdcage coil 70) at thetime of RF signal transmission. The details of this operation will bedescribed later.

Immediately before a RF magnetic field having the resonance frequency f₁or f₂ is applied from the RF magnetic field generator 106 to thedouble-tuned birdcage coil 70, the magnetic coupling preventing circuitdriver 115 sets the value of the control current 51 to be flown throughthe PIN diode 30 of the double-tuned birdcage coil 70 to be 0, andapplies a direct control current 51 so that the PIN diode 31 of thedouble-tuned loop-type coil 71 should be turned on.

By applying the control current 51 to the double-tuned loop-type coil71, the PIN diode 31 is turned on, the parallel resonant circuit 55having the capacitor 26 and the inductor 16 resonates at the resonancefrequency f₁, and the parallel resonant circuit 56 having the capacitor27 and the inductor 17 resonates at the resonance frequency f₂. As aresult, impedance of the double-tuned loop-type coil 71 becomesextremely high, thus current hardly flows through the double-tunedloop-type coil 71, and magnetic field is hardly generated, either.

On the other hand, since the value of the control current 51 which flowsthrough the diode 30 becomes 0 in the double-tuned birdcage coil 70, allthe diodes 30 are turned off, the parallel resonant circuit 54 comes tobe a circuit equivalent to a parallel circuit including the circuithaving two of the capacitors 26 and 27 connected in series and theinductor 18 connected in parallel (trap circuit), and the double-tunedbirdcage coil 70 resonates at the resonance frequency f₁ and theresonance frequency f₂.

Therefore, magnetic coupling of the double-tuned birdcage coil 70 andthe double-tuned loop-type coil 71 is eliminated, and the double-tunedbirdcage coil 70 can irradiate a RF magnetic field of the resonancefrequency f₁ or f₂ on the subject 103 without shift of the resonancefrequency or fall of the Q value of the coil due to magnetic coupling.

When magnetic resonance signals emitted from the subject 103 arereceived after applying the RF magnetic field, the magnetic couplingpreventing circuit driver 115 applies the control current 51 so that thediode 30 of the double-tuned birdcage coil 70 should be turned on, andsets the value of the control current 51 flown through the diode 31 ofthe double-tuned loop-type coil 71 to be 0.

When the control current 51 is applied to the double-tuned birdcage coil70, the diode 30 is turned on, the parallel resonant circuit 130 havingthe capacitor 26 and the inductor 16 resonates at the resonancefrequency f₁, and the parallel resonant circuit 131 having the capacitor27 and the inductor 17 resonates at the resonance frequency f₂. As aresult, impedance of the double-tuned birdcage coil 70 becomes extremelyhigh at the resonance frequencies f₁ and f₂, current hardly flows in thedouble-tuned birdcage coil 70, and magnetic field is hardly generated,either.

On the other hand, since the value of the control current 51 which flowsthrough the diode 31 becomes 0 in the double-tuned loop-type coil 71,the diode 31 is turned off, and the connection between the inductor 16and the capacitor 26 and the connection between the inductor 17 and thecapacitor 27 are cut off. As a result, the double-tuned loop-type coil71 becomes a circuit equivalent to the double-tuned loop-type coil 150of the first embodiment, and operates as a coil which resonates at theresonance frequencies f₁ and f₂.

Therefore, when two kinds of magnetic resonance signals corresponding tothe resonance frequencies f₁ and f₂ emitted from the subject 103 arereceived, magnetic coupling of the double-tuned birdcage coil 70 and thedouble-tuned loop-type coil 71 is eliminated, and the double-tunedloop-type coil 71 receives magnetic resonance signals corresponding tothe resonance frequency f₁ or f₂ with high sensitivity without shift ofthe resonance frequency or fall of the Q value of the coil due tomagnetic coupling.

As described above, according to this embodiment, magnetic coupling ofthe double-tuned birdcage RF coil 70 and the double-tuned loop-type coil71 which are tuned to two kinds of magnetic resonance frequencies closeto each other can be prevented at the time of application of a RFmagnetic field and reception of magnetic resonance signals. As a result,the double-tuned birdcage RF coil 70 can transmit signals of a uniformRF magnetic field having two kinds of magnetic resonance frequenciesclose to each other, and the double-tuned loop-type coil 71 cansimultaneously receive magnetic resonance signals of two kinds offrequencies close to each other with high sensitivity.

Therefore, according to this embodiment, the shape of the transmit RFcoil 116 a and the shape of receive RF coil 116 b can be independentlychosen. According to this embodiment, in addition to the effect of thefirst embodiment, the effect brought by the shape of RF coil 116 can beobtained. For example, by using a double-tuned birdcage coil 70 showinghigh uniformity of irradiation distribution as the transmit RF coil 116a and selecting the shape of the receive RF coil 116 b according to theshape and size of the subject 103, magnetic resonance images optimizedfor each individual subject 103 can be obtained. Of course, the transmitRF coil 116 a is not limited to the double-tuned birdcage RF coil 70.

In this embodiment, a double-tuned array coil 72 such as shown in FIG.17 can be used as the receive RF coil 116 b. The double-tuned array coil72 is made up of a plurality of partially overlapping loop coils 1 (4coils in FIG. 17). The positions of the overlapping portions of theadjacent loop coils 1 are adjusted so that magnetic coupling of the loopcoils 1 should be eliminated. By using the double-tuned array coil 72,it becomes possible to image a region wider than that can be imaged byusing one double-tuned receive coil 71. Therefore, it becomes possibleto, for example, simultaneously receive magnetic resonance signals oftwo kinds of frequencies close to each other with high sensitivity forthe whole trunks of a test person (patient) as the subject 103.

In addition, the configuration of the magnetic coupling preventingcircuit 68 is not limited to that mentioned above. For example, it mayhave the configuration shown in FIG. 16C. The magnetic couplingpreventing circuit 69 shown in FIG. 16C is provided with a cross diode34 having a combination of two PIN diodes of which polarities areinverse each other instead of the magnetic coupling preventing circuitdriver 115 and the PIN diode driven thereby provided in the magneticcoupling circuit 68. The capacitor 26 and the capacitor 27 are connectedin series. A circuit having the cross diode 34 and the inductor 16connected in series is connected to the capacitor 26 in parallel, and acircuit having the cross diode 34 and the inductor 17 connected inseries is connected to the capacitor 27 in parallel.

Immediately before a RF magnetic field having the resonance frequency f₁or f₂ is applied from the RF magnetic field generator 106 to thedouble-tuned birdcage coil 70, the magnetic coupling preventing circuitdriver 115 sets the value of the control current 51 to be flown throughthe PIN diode 30 of the double-tuned birdcage coil 70 to be 0.

Since the value of the control current 51 which flows through the diode30 becomes 0 in the double-tuned birdcage coil 70, all the diodes 30 areturned off, the parallel resonant circuit 54 comes to be a circuitequivalent to the parallel circuit including the circuit having two ofthe capacitors 26 and 27 connected in series and the inductor 18connected in parallel (trap circuit), and the double-tuned birdcage coil70 resonates at the resonance frequency f₁ and the resonance frequencyf₂.

On the other hand, intense electromotive force is generated by magneticcoupling in the double-tuned loop-type coil 71 to which the RF magneticfield is applied, and the cross diode 34 is turned on. The parallelresonant circuit 57 having the capacitor 26 and the inductor 16resonates at the resonance frequency f₁, and the parallel resonantcircuit 58 comprising the capacitor 27 and the inductor 17 resonates atthe resonance frequency f₂. As a result, impedance of the double-tunedloop-type coil 71 becomes extremely high, thus current hardly flowsthrough the double-tuned loop-type coil 71, and magnetic field is hardlygenerated, either.

Therefore, magnetic coupling of the double-tuned birdcage coil 70 andthe double-tuned loop-type coil 71 is eliminated, and the double-tunedbirdcage coil 70 can irradiate a RF magnetic field of the resonancefrequency f₁ or f₂ on the subject 103 without shift of the resonancefrequency or fall of the Q value of the coil due to magnetic coupling.

When magnetic resonance signals emitted from the subject 103 arereceived after applying the RF magnetic field, the magnetic couplingpreventing circuit driver 115 applies the control current 51 to be flownin the PIN diode 30 of the double-tuned birdcage coil 70.

When the control current 51 is applied to the double-tuned birdcage coil70, the diode 30 is turned on, the parallel resonant circuit 130 havingthe capacitor 26 and the inductor 16 resonates at the resonancefrequency f₁, and the parallel resonant circuit 131 having the capacitor27 and the inductor 17 resonates at the resonance frequency f₂. As aresult, impedance of the double-tuned birdcage coil 70 becomes extremelyhigh at the resonance frequencies f₁ and f₂, current hardly flows in thedouble-tuned birdcage coil 70, and magnetic field is hardly generated,either.

On the other hand, the double-tuned loop-type coil 71 receives magneticresonance signals generated by the subject 103. However, since themagnetic resonance signals have extremely small currents, the crossdiode 34 is turned off, and is not connected to the inductor 16 and theinductor 17. As a result, the double-tuned loop-type coil 71 becomes acircuit equivalent to the double-tuned loop-type coil 150 of the firstembodiment, and operates as a coil which resonates at the resonancefrequencies f₁ and f₂.

As described above, when the magnetic coupling preventing circuit 69 isused, magnetic coupling of double-tuned birdcage transmit RF coil 70 andthe double-tuned receive coil 71 can be prevented without using themagnetic coupling preventing circuit driver 115 in the double-tunedreceive coil. Therefore, in addition to the effect obtained in the caseof using the magnetic coupling preventing circuit 68, an effect that theconfiguration can further be simplified can also be obtained.

Fourth Embodiment

Hereafter, the fourth embodiment of the present invention will beexplained. The configuration of the MRI apparatus based on thisembodiment is basically the same as those of the aforementionedembodiments. In this embodiment, as the transmit and receive RF coil116, a combination of two of double-tuned loop-type coils 150 of thefirst embodiment is used as in the second embodiment. In thisembodiment, however, two of the double-tuned loop-type coils aredisposed so that the loop planes of the loop coils should be in the sameplane. The configuration of this embodiment different from that of thesecond embodiment will be explained below.

FIG. 18 is a diagram for explaining a transmit and receive RF coil 116of this embodiment. In the diagram, the direction of the z axis in thecoordinate 9 is the static magnetic field direction. As shown in thisdiagram, the transmit and receive RF coil 116 of this embodiment isprovided with a first double-tuned loop-type coil 59 and a seconddouble-tuned loop-type coil 60. The configurations of the double-tunedloop-type coils 59 and 60 are the same as that of the double-tunedloop-type coil 150 of the first embodiment. Furthermore, as in thedouble-tuned loop-type coil 150 of the first embodiment, the firstdouble-tuned loop-type coil 59 and the second double-tuned loop-typecoil 60 of this embodiment are adjusted so that they should resonate attwo different resonance frequencies.

The first double-tuned loop-type coil 59 of the transmit and receive RFcoil 116 of this embodiment is disposed so that the loop plane 173thereof should be in a plane parallel to the xz plane of the coordinate9. The second double-tuned loop-type coil 60 is disposed so that theloop plane 174 thereof should be in the same plane as the loop plane 173of the first double-tuned loop-type coil 59. The second double-tunedloop-type coil 60 is disposed inside the first double-tuned loop-typecoil 59, and it is adjusted so as to resonate at two resonancefrequencies which are different from those of the first double-tunedloop-type coil 59. A parallel resonant circuit 35 and a parallelresonant circuit 36, which are adjusted for the two frequencies to whichthe second double-tuned loop-type coil 60 is tuned, respectively, areinserted into the loop coil of the first double-tuned loop-type coil 59in series in order to prevent magnetic coupling thereof with the seconddouble-tuned loop-type coil 60. On the other hand, a parallel resonantcircuit 37 and a parallel resonant circuit 38, which are adjusted forthe two frequencies to which the first double-tuned loop-type coil 59 istuned, respectively, are inserted into the loop coil of the seconddouble-tuned loop-type coil 60 in series in order to prevent magneticcoupling thereof with the first double-tuned loop-type coil 59.

The operation of the transmit and receive RF coil 116 of this embodimentwill be explained below. Explained here as an example is a case whereinductors 11, 12 and 13 and capacitors 21, 22 and 23 of the firstdouble-tuned loop-type coil 59 are adjusted so as to resonate at thefrequencies of ¹H and ¹⁹F (they are defined to be the first resonancefrequency f₁ and the second resonance frequency f₂, respectively), andinductors 81, 82 and 83 and capacitors 91, 92 and 93 of the seconddouble-tuned loop-type coil 60 are adjusted so as to resonate at thefrequencies of ²³Na (sodium) and ¹³C (carbon) (they are defined to bethe third resonance frequency f₃ and the fourth resonance frequency f₄,respectively).

When a signal of the first resonance frequency f₁ is transmitted to thefirst double-tuned loop-type coil 59, or when the first double-tunedloop-type coil 59 receives a signal of the first resonance frequency f₁(hereafter transmission and reception are collectively referred to “tobe tuned”), since the parallel resonant circuit 37 inserted into theloop coil of the second double-tuned loop-type coil 60 in series isadjusted so as to resonate at the first resonance frequency f₁, theimpedance thereof becomes extremely high. On the other hand, when thefirst double-tuned loop-type coil 59 is tuned to the second resonancefrequency f₂, since the parallel resonant circuit 38 inserted into theloop of the second double-tuned loop-type coil 60 in series is adjustedso as to resonate at the second resonance frequency f₂, the impedancethereof becomes extremely high. Therefore, even when the firstdouble-tuned loop-type coil 59 is tuned to whichever frequency (f₁ orf₂), magnetic coupling of the first double-tuned loop-type coil 59 andthe second double-tuned loop-type coil 60 is eliminated, and thus thefirst double-tuned loop-type coil 59 can irradiate a RF magnetic fieldof the first or second resonance frequency (f₁ or f₂) on the subject 103and detect signals from the subject 103 without shift of the resonancefrequency or fall of the Q value of the coil due to magnetic coupling.

Similarly, when the second double-tuned loop-type coil 60 is tuned tothe third resonance frequency f₃, since the parallel resonant circuit 35inserted into the loop of the first double-tuned loop-type coil 59 inseries is adjusted so as to resonate at the third resonance frequencyf₃, the impedance thereof becomes extremely high. On the other hand,when the second double-tuned loop-type coil 60 is tuned to the fourthresonance frequency f₄, since the parallel resonant circuit 36 insertedinto the loop of the first double-tuned loop-type coil 59 in series isadjusted so as to resonate at the fourth resonance frequency, theimpedance thereof becomes extremely high. Therefore, even when thesecond double-tuned loop-type coil 60 is tuned to whichever frequency(f₃ or f₄), magnetic coupling of the first double-tuned loop-type coil59 and the second double-tuned loop-type coil 60 is eliminated, and thusthe second double-tuned loop-type coil 60 can irradiate a RF magneticfield of the third or fourth resonance frequency (f₃ or f₄) on thesubject 103 and detect signals from the subject without shift of theresonance frequency or decrease of the Q value of the coil due tomagnetic coupling.

As described above, according to this embodiment, magnetic coupling ofthe double-tuned loop-type coil 59 and the double-tuned loop-type coil60, which are tuned to two magnetic resonance frequencies close to eachother, respectively, can be prevented at the time of application of a RFmagnetic field and reception of magnetic resonance signals. Therefore,according to this embodiment, it becomes possible to obtain signals offour resonance frequencies, and in addition to the effect of the firstembodiment, imaging of more various nuclides becomes possible withoutexchanging the coil.

In addition, in the aforementioned embodiments, the second seriesresonant circuit 42, the third series resonant circuit 43 and the signalprocessing circuit 45 may be covered with an electric wave shield 52.The configuration and operation in the case of covering with theelectric wave shield 52 will be explained below for the case of thedouble-tuned loop-type coil 150 of the first embodiment as an example.

FIG. 19 is a diagram for explaining the case of applying the electricwave shield 52 to the double-tuned loop-type coil of the firstembodiment. As shown in this diagram, in this case, the second seriesresonant circuit 42, the third series resonant circuit 43 and the signalprocessing circuit 45 of the double-tuned loop-type coil 150 are coveredwith the electric wave shield 52. Further, the electric wave shield 52is grounded on the earth. The signal processing circuit 45 is connectedto the signal wire 53.

Because the second series resonant circuit 42, the third series resonantcircuit 43 and the signal processing circuit 45 are covered with theelectric wave shield 52, influence of the RF magnetic field generated bythe loops of the portions of the second series resonant circuit 42, thethird series resonant circuit 43 and the signal processing circuit 45 onthe RF magnetic field generated by the loop coil 1 can be reduced.Therefore, according to this embodiment, a RF magnetic field can beirradiated on the subject 103 with suppressing turbulence of themagnetic field generated by the loop coil 1. Further, magnetic couplingof the second series resonant circuit 42, the third series resonantcircuit 43 and the signal processing circuit 45 with the subject 103 canbe prevented by the electric wave shield 52. That is, according to thisembodiment, influence of external noises can be reduced, and loss due tomagnetic coupling can be reduced.

1. A RF coil for magnetic resonance imaging apparatus, tuned to at leasttwo different resonant frequencies, comprising: a first series resonantcircuit comprising a loop coil made of a conductive material and one ormore capacitors inserted into the loop coil, a first circuit connectedin parallel to the first series resonant circuit, and a signalprocessing circuit connected in parallel to the first circuit, andhaving two or more different resonance frequencies, wherein: the firstcircuit comprises one or more capacitors and one or more inductors, andis connected in parallel with at least two series resonant circuits eachhaving different resonance frequencies, the resonance frequencies of theat least two series resonant circuits also differ from resonancefrequency of the first series resonant circuit, and each of theresonance frequencies of the RF coil are adjusted so as to be betweenthe resonance frequency of the first series resonant circuit and theresonance frequencies of the at least two series resonant circuits.
 2. ARF coil for magnetic resonance imaging apparatus, tuned to at least twodifferent resonant frequencies, comprising: a first series resonantcircuit comprising a loop coil made of a conductive material and one ormore capacitors inserted into the loop coil, a second series resonantcircuit comprising one or more capacitors and one or more inductorsconnected in series and connected in parallel to the first seriesresonant circuit, a third series resonant circuit comprising one or morecapacitors and one or more inductors connected in series and connectedin parallel to the second series resonant circuit, and a signalprocessing circuit connected in parallel to the third circuit, wherein:resonance frequency f_(A) of the first series resonant circuit,resonance frequency f_(B) of the second series resonant circuit,resonance frequency f_(C) of the third series resonant circuit, firstresonance frequency f₁ and second resonance frequency f₂ of the RF coilare adjusted so as to satisfy a relation of f_(B)<f₁<f_(A)<f₂<f_(C). 3.The RF coil according to claim 2, wherein the loop coil comprises twoloops of a conductive material disposed in a shape of cylindrical sideso as to face each other in a saddle shape and connected so thatdirections of magnetic fields generated by the loops of a conductivematerial should be the same.
 4. The RF coil according to claim 2,wherein the loop coil comprises two loops of a conductive materialadjacently disposed in the same plane in a butterfly shape and connectedso that directions of magnetic fields generated by the loops of aconductive material should be inverse to each other.
 5. The RF coilaccording to claim 2, wherein the loop coil has a solenoid shape.
 6. TheRF coil according to claim 2, which further comprises: a fourth seriesresonant circuit connected in parallel between the third series resonantcircuit and the signal processing circuit, wherein: the resonancefrequencies f_(A), f_(B), f_(C), f₁, f₂, resonance frequency f_(p) ofthe fourth series resonant circuit, and third resonance frequency f₃(>f₂>f₁) of the RF coil are adjusted so as to satisfy a relation off_(B)<f₁<f_(A)<f₂<f_(C)<f₃<f_(D) orf_(B)<f₁<f_(C)<f₂<f_(A)<f₃<f_(D).
 7. The RF coil according to claim 2,wherein portions of the coil other than that of the first seriesresonant circuit are provided with electric wave shield.
 8. The RF coilaccording to claim 2, wherein the first resonance frequency correspondsto 70% or more of the second resonance frequency.
 9. A RF coil systemfor magnetic resonance imaging apparatus comprising: a first RF coil,and a second RF coil, wherein: the first RF coil is the RF coilaccording to claim 2, the second RF coil is the RF coil according toclaim 2, the coils are disposed so that direction of magnetic fieldgenerated by the second RF coil should be perpendicular to direction ofmagnetic field generated by the first RF coil, and a phase of signalapplied to the second RF coil is different from a phase of signalsapplied to the first RF coil by 90 degrees.
 10. A RF coil system formagnetic resonance imaging apparatus comprising: an array coilcomprising two or more of the RF coils according to claim 2 disposed sothat the loop coil portions should partially overlap.
 11. A RF coilsystem for magnetic resonance imaging apparatus comprising: a magneticfield transmit RF coil, a magnetic field receive RF coil, and a magneticcoupling preventing means, wherein: the magnetic field transmit RF coilcomprises the RF coil that operate at different resonance frequencies,the magnetic field receive RF coil is the RF coil according to claim 2,and the magnetic coupling preventing means makes the magnetic fieldreceive coil to be in an open state at a time of transmission of signalsof the first resonance frequency and the second resonance frequency, andmakes the magnetic field transmit coil to be in an open state at a timeof reception of signals of the first resonance frequency and the secondresonance frequency.
 12. The RF coil system according to claim 11,wherein the magnetic coupling preventing means comprises a diode, andthe diode is turned on and off with an external control signal.
 13. TheRF coil system according to claim 11, wherein the magnetic couplingpreventing means comprises a cross diode comprising two diodes connectedin reverse directions.
 14. A RF coil system for magnetic resonanceimaging apparatus comprising: a first magnetic field transmit andreceive RF coil, a second magnetic field transmit and receive RF coil,and a magnetic coupling preventing means, wherein: the first magneticfield transmit and receive RF coil and the second magnetic fieldtransmit and receive RF coil are the RF coils according to claim 2, twoof the resonance frequencies of the second magnetic field transmit andreceive RF coil are adjusted so as to be different from two of theresonance frequencies of the first magnetic field transmit and receiveRF coil, and the magnetic coupling preventing means makes the secondmagnetic field transmit and receive RF coil to be in an open state at atime of transmission and reception of signals of two of the resonancefrequencies of the first magnetic field transmit and receive RF coil,and makes the first magnetic field transmit and receive RF coil to be inan open state at a time of transmission and reception of signals of twoof the resonance frequencies of the second magnetic field transmit andreceive RF coil.
 15. A magnetic resonance imaging apparatus comprising astatic magnetic field generating means for generating a static magneticfield, a gradient magnetic field generating means for generating agradient magnetic field, a RF magnetic field generating means forgenerating a RF magnetic field, a transmit and receive RF coil forapplying the RF magnetic field to a subject and receiving magneticresonance signals from the subject, and a control means for controllingthe gradient magnetic field generating means, the RF magnetic fieldgenerating means and the transmit and receive RF coil, wherein thetransmit and receive RF coil is the RE coil according to claim
 2. 16. Amagnetic resonance imaging apparatus comprising a static magnetic fieldgenerating means for generating a static magnetic field, a gradientmagnetic field generating means for generating a gradient magneticfield, a RF magnetic field generating means for generating a RF magneticfield, a transmit coil for applying the RF magnetic field to a subject,a receive coil for receiving magnetic resonance signals from thesubject, and a control means for controlling the gradient magnetic fieldgenerating means, the RE magnetic field generating means, the transmitcoil and the receive coil, wherein the transmit coil is the RF coilaccording to claim
 2. 17. A magnetic resonance imaging apparatuscomprising a static magnetic field generating means for generating astatic magnetic field, a gradient magnetic field generating means forgenerating a gradient magnetic field, a RF magnetic field generatingmeans for generating a RF magnetic field, a transmit coil for applyingthe RF magnetic field to a subject, a receive coil for receivingmagnetic resonance signals from the subject, and a control means forcontrolling the gradient magnetic field generating means, the RFmagnetic field generating means, the transmit coil and the receive coil,wherein the receive coil is the RF coil according to claim 2.